NASA's acting chief Robert Lightfoot is set to retire at the end of April, even as US President Donald Trump's permanent choice to lead the space agency remains stuck in Congress, the media reported.
Lightfoot, who started in 1989 at the Marshall Centre in Huntsville, Alabama, as a test engineer, has been the acting administrator since the beginning of the Trump administration in January 2017 for nearly 14 months.
His tenure would have been considerably shorter but the Senate has yet to confirm Trump nominee James Bridenstine, a Republican congressman from Oklahoma, the USA Today reported.
"I cannot express enough my gratitude to the entire NASA team for the support during my career and especially the last 14 months as your acting administrator," Lightfoot said in a "bittersweet" goodbye note sent to the agency staff on Monday.
"The grit and determination you all demonstrate every day in achieving our missions of discovery and exploration are simply awe inspiring.
"I leave NASA blessed with a career full of memories of stunning missions, cherished friendships, and an incredible hope for what is yet to come," Lightfoot said.
Lightfoot, further said that he plans to work with the White House "on a smooth transition to the new administrator".
His retiement adds more uncertainty about the agency's leadership.
The White House nominated Bridenstine to fill NASA's top post in September 2017 and he resubmitted the nomination in January.
In both cases his nomination has been advanced by the Senate Commerce Committee on a strict party-line vote, but Bridenstine lacks the 50 votes needed to be confirmed.
Bridenstine is a commercial space advocate who has been endorsed by prominent astronauts such as Apollo 11's Buzz Aldrin.
Strongly opposing Bridenstine, Democratic Party's Florida Senator Bill Nelson, said that he is a divisive figure whose presence could impede the bipartisan support necessary for NASA's long-term agenda to expand space exploration,
On the other hand, Members of the House Science Committee has criticised the Senate for not taking action on Bridenstine's nomination, the report said.
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